1. Field of the Invention
This invention relates to a device and method for measuring the level of liquid in a container. More particularly, the invention relates to a device and method for sensing and quantifying the level of liquid in a container in liquid handling equipment for medical devices.
2. Discussion of the Art
Liquid level sensors, particularly liquid level sensors that determine the level of liquid in a container by measurement of capacitance, are widely used to detect the heights of liquids (liquid levels) inside containers. Many automated analytical instruments utilize liquid level sensors. For example, liquid level sensors are used in conjunction with the m2000 systems commercially available from Abbott Molecular Diagnostics Division, Abbott Laboratories, Des Plaines, Ill., for sample preparation for polymerase chain reaction (alternatively referred to herein as “PCR”). Erroneous liquid level measurements may cause incorrect and invalid analytical results in assays, and, consequently, adversely affect the reliability of systems utilizing automated analytical instruments.
U.S. Pat. Nos. 3,635,094; 4,326,851; 4,736,638; 4,912,976; 5,049,826; 5,275,951; 5,365,783; 5,627,522; 5,639,426; 5,648,727; 5,885,8516, 148,666; 6,270,726; EP 0 633 456; and WO 2005/0456635 describe liquid level sensors that employ capacitance measurements to determine the level of a liquid in a container.
In liquid level sensors utilizing capacitance measurements, typically an electrically conductive tip of a probe approaches a container containing a liquid. An electrical signal, such as, for example, a signal that correlates with the impedance determined by the liquid level sensing apparatus, is measured. From well-known capacitance equations, it is known that capacitance values depend on the distance of the electrically conductive tip of the probe from the surface of the liquid. Also, because the dielectric constants of air and water (or aqueous solutions) are markedly different, the capacitance value undergoes a sudden change as the electrically conductive tip of the probe makes the transition from air to contact the surface of the liquid. Furthermore, after the electrically conductive tip of the probe enters the liquid, the capacitance varies as a function of the height of the liquid between the electrically conductive tip of the probe and the other electrode in the capacitance system. The other electrode in the system is usually the ground plane. See, for example, U.S. Pat. No. 5,648,727. The capacitance value increases as the distance separating the electrodes decreases.
In a conventional capacitor, two metallic plates are separated by a layer of insulating material. In liquid level measurements, the capacitor employed comprises the electrically conductive tip of the probe as one “plate” and the instrument ground as the second “plate”.
Electrical capacitance (capacitance) refers to the ability to store an electrical charge. The classical representation of a capacitor involves two parallel plates separated by an air gap or an insulator. In such a configuration, the capacitance is defined as:
                                                        C              =                              Q                ⁢                                  /                                ⁢                V                                                                                        =                              A                ⁢                                                                  ⁢                                  ɛ                  r                                ⁢                                  ɛ                  o                                ⁢                V                ⁢                                  /                                ⁢                d                                                                        (        1        )            where:                C represents capacitance        V represents voltage        Q represents charge        A represents overlapping surface area (effective area) of the electrical conductors        ∈o represents permittivity of free space (constant)        ∈r represents relative dielectric constant of the insulating material        d represents distance between the electrical conductors        
Of particular note is the dependence of the capacitance, as defined in Equation (1), on the spacing (represented by “d”) and area (represented by “A”) of the electrical conductors, the electrical characteristic of the gap insulating material (represented by the relative dielectric constant, ∈r), as capacitance increases as ∈r increases. It is also important to note that “A” represents an “effective” area, representing the overlap of the two plates, for the overlapping area defines the electric field.
In many applications, the capacitor is connected to a radio frequency (RF) electrical signal. For sensing levels of liquids, the electrodes of the capacitor are usually connected to the feedback loop of an RF oscillator (frequencies in the range of ˜30 KHz to ˜1 MHz). In this case, the charging and discharging behavior of the capacitor, in addition to the factors enumerated above, will depend on the frequency of the RF signal. It is more practical to describe the impedance of the system, Z, which is a complex function defining the total “resistance” to current flow:Z(f)=R+1/(2 π j f C)  (2)                where:        R represents resistance        C represents capacitance        f represents measurement frequency (RF)        π represents constant (3.14159 . . . )        j=√{square root over (−1)}or,Z(f)=R+1/(k ′f C)   (3)        
where k′ combines the constants in Equation (2), i.e., k′=2 π j.
It should be noted that equation (2) for impedance also includes the resistance (R). The value of resistance is significant where, for example, the electrically conductive tip of a pipette (i.e., the electrically conductive tip of the probe) makes the transition from air to the surface of a liquid, leading to a marked change in (R), because most solutions are more electrically conductive than air. Methods of liquid level sensing based on RF impedance are commonly provided with specific circuitry that measures changes in capacitance resulting from changes in the level of liquid in a container. Such circuitry can have numerous variations. Representative examples of such circuitry is described, for example, in U.S. Pat. Nos. 3,635,094; 4,326,851; 4,736,638; 5,049,826; 5,275,951; 5,365,783; 5,627,522; 5,648,727; 6,148,666; 6,270,726; EP 0 633 456; and WO 2005/0456635, all of which are incorporated herein by reference to illustrate and describe circuitry suitable for use in measuring changes in capacitance by means of a liquid level sensor.
Measurements of liquid level in liquid handling systems by means of capacitance, such as for example, the m1000 and m2000sp instruments from Abbott Molecular Diagnostics (a division of Abbott Laboratories), Des Plaines, Ill., typically use the metallic surface of the deck of the instrument as one of the plates of the capacitor, the other plate of the capacitor being the electrically conductive tip of the probe. The electrically conductive tip of the probe acts as an electrode, and is connected to a RF source, while the deck of the analytical instrument acts as another electrode. In this configuration, errors in determining liquid level occur frequently. Measurement of the electrical signal representing capacitance varies with the height of the electrically conductive tip of the probe. However, measurements show that there is an excessive amount of noise in the electrical signal. Noise is indicated by the standard deviation in repeated measurements. It is believed that the noise occurs because of a poorly defined capacitor configuration when the large surface of the instrument (e.g., surface area greater than 5000 cm2) is used as a capacitor plate. Additionally, such a configuration may allow interference from neighboring components on the deck of the instrument.
The prior art cited describe liquid level sensing, primarily emphasizing liquid level sensing by means of capacitance. It has been observed that in many instruments, the capacitors used for liquid level measurement are not constructed to reliably perform liquid level sensing. The poor construction can be attributed to unconventional components of the capacitor used for liquid level measurements, i.e., the electrically conductive tip of the probe and the electrically conductive deck of the instrument.
It would be desirable to provide methods to improve the reliability and accuracy of determining the level of liquid in a container in an automated instrument.